Thx QuietNose, but var $targets = $('#'+item.field_id); broke all my conditionals if no repeater fields are present. It’s because the attribute data-field_id is empty on those fields.
I don’t dive deep into the code, but with the following dirty hack I go around the issue: var $targets = (item.field_id.length != 0) ? $('#'+item.field_id) : $('.field_key-' + item.field);
Only tested with repeater add-on!
